1. Field of the Invention
The present invention relates to a neuromorphic device imitating a synapse, a synapse array, or a neuron in a neuromorphic technique.
2. Description of the Related Art
Recently, in integrated circuits based on a Von Neumann architecture, power consumption has been greatly increased, and the problem of heat release has been very serious. Therefore, in the field of Von-Neumann-based integrated circuits, many approaches to imitate nervous systems of animals have been studied. In particular, in techniques of imitating the nervous systems of animals, the power consumption can be greatly reduced, and perceiving and learning are enabled, so that perception and determination functions can be improved. Accordingly, the functions of existing Von-Neumann-based integrated circuits are expected to be greatly improved or replaced by using the techniques of imitating the nervous systems of animals. Therefore, much attention has been increasingly paid on the techniques of imitating the nervous systems of animals, and there is much need to study techniques of imitating the nervous systems of animals.
A basic function of a neuron is to generate an electrical spike (signal) from a stimulus exceeding a threshold value and transmit information to another cell. The generated electrical signal is called an action potential. A neuron may be mainly divided into three portions. Namely, the neuron includes a nerve cell body where a nucleus exists, a dendrite which receives a signal from another cell, and an axon which transmits a signal to another cell. A portion which transmits a signal between the dendrites is called a synapse.
The neuron receives a stimulus from another nerve cell or stimulus receptor cell and transmits the stimulus to another nerve cell or glandular cell. Exchanging stimuli occurs in the synapse. One nerve cell (neuron) receives stimuli through a plurality of the synapses to integrate excitations, and after that, the nerve cell transmits an electrical spike to an axon near to the nerve cell body, so that the electrical spike reaches the synapse.
In this manner, the transmission of excitations from the neuron through the synapse to another nerve cell is referred to as excitation transmitting. The excitation in the synapse is transmitted only in the direction from a nerve fiber to a nerve cell body or a dendrite and is not transmitted in the reverse direction. As a whole, the synapse transmits the excitation only in the one direction.
In addition, the synapse is a relay site of transmitting the excitations, and the synapse applies a weighting factor according to temporal or spatial change of excitations reaching the synapse or make inhibition to enable a high-level integrating function of the nerve system.
On the other hand, besides the synapse which transmits the excitations, there are some synapses having a function of inhibiting the excitations of the nerve cell. The synapse having the function is called an inhibitory synapse. When the excitation transmitted along nerve fiber reaches the inhibitory synapse, the synapse secretes an inhibitory transmitting material. The inhibitory material acts on a membrane of the nerve cell connected to the synapse to inhibit excitations of the cell from occurring (occurrence of an action potential). As a result, while the inhibitory transmitting material acts, the excitation reaching another synapse is not transmitted to the synapse.
In this manner, the neuron performs an excitation transmitting function of transmitting excitations received from one or more nerve cells through the synapse to another nerve cell, an excitation integrating/transmitting function of integrating excitations received from a plurality of nerve cells and transmitting an integrated excitation to another nerve cell, or an excitation inhibiting function of inhibiting an excitation from be transmitted from another nerve cell.
The present invention is to provide a neuromorphic device capable of imitating various functions such as excitation integrating, transmitting, and inhibiting functions performed by the neuron and the synapse.
In the related art, there are mainly memrister-based techniques and SRAM-based techniques. In the memrister-based techniques, an existing RRAM or PRAM is mainly used, and in some cases, an STT-MRAM is used. Since two-terminal elements are basically used, these techniques seem to be simple. However, in implementation of an actual synapse array, complexity of interconnection for the excitatory or inhibitory functionality is greatly increased. In addition, there is a problem in that, as a limitation of the two-terminal element in an array environment, a selection element needs to be installed in each neuromorphic device. In the case of using the RRAM or the PRAM, an analog memory suitable for imitating the functions of the synapse can be implemented. However, in the case of using the SRAM, since the SRAM is a digital memory, there is a limitation to implement one synapse. In particular, the case of using the RRAM has a serious problem in durability, and the case of using the PRAM has a problem in durability and a problem according to high power consumption. Since an existing SRAM cell for imitating the synapse is typically configured with eight transistors, the occupied area is too large, and there is a limitation as a digital memory described above. Therefore, there is a serious problem in degree of integration.
US Patent Laid-Open Nos. US 2014-0129498 and US 2012-0084241 are disclosed as the related art.
Nanoscale Memristor Device as Synapse in Neuromorphic Systems (Nano Lett. 10, 4(2010)) and Phase Change Memory as Synapse for Ultra-Dense Neuromorphic Systems: Application to Complex Visual Pattern Extraction (IEEE IEDM (2011)) are disclosed as the related art.